14: DNA Replication
- Page ID
- 42832
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- 14.1: DNA Structure
- By 1878, a substance in the pus of wounded soldiers derived from cell nuclei (called nuclein) was shown to be composed of 5 bases (the familiar ones of DNA and RNA). The four bases known to make up DNA (as part of nucleotides) were thought to be connected through the phosphate groups in short repeating chains of four nucleotides. By the 1940s, we knew that DNA was a long polymer. Nevertheless, it was still considered too simple to account for genes.
- 14.2: Genes and Chromatin in Eukaryotes
- Chromosomes and chromatin are a uniquely eukaryotic association of DNA with more or less protein. Bacterial DNA (and prokaryotic DNA generally) is relatively ‘naked’ – not visibly associated with protein. The electron micrograph of an interphase cell (below) reveals that the chromatin can itself exist in various states of condensation.
- 14.3: Introduction
- Replication begins at one or more origins of replication along DNA, where helicase enzymes catalyze unwinding of the double helix. DNA unwinding creates replicating bubbles, or replicons, with replication forks at either end. Making a new DNA strand starts with making an RNA primer with RNA nucleotides and primase enzymes.
- 14.4: DNA Replication
- As we’ve seen, DNA strands have directionality, with a 5’ nucleotide-phosphate and a 3’ deoxyribose hydroxyl end. This is even true for circular bacterial chromosomes…, if the circle is broken! Because the strands of the double helix are antiparallel, the 5’ end of one strand aligns with the 3’end of the other at both ends of the double helix. The complementary pairing of bases in DNA means that the base sequence of one strand can be used as a template to make a new complementary strand. As we’l